In large high-performance very large scale integration (VLSI) chips, an internal clock signal is distributed throughout the chip to control timing of the chip as a function of an external system clock. Both the clock signals include a rising edge and a falling edge every clock cycle. The internal clock cycle time comprises several components, such as delays associated with storage devices on the chip, clock skew, logic evaluation, and signal transmission. Of these four components only the logic evaluation component performs real work, the other three components are overhead that merely add to the cycle time.
The internal clock signal is typically generated from the external clock by a circuit called a clock buffer, and then distributed to the circuits in the chip through some form of on-chip clock distribution network. Ordinarily, the clock buffer includes a large inverter that receives the external clock signal and transmits a modified signal to two output transistors, which transmit the internal clock signal to the on-chip clock distribution network.
It is advantageous to have the clock signal transition between voltage and ground as fast as possible. This entails producing an edge as fast as possible, while maintaining the time the edge rises or falls during each clock cycle.
During the transition of the clock edge, the inverter momentarily diverts the current intended to drive the clock distribution network through the two output transistors by briefly turning both devices on. This is disadvantageous since this current degrades the transition time of the clock edge by diverting current away from the clock distribution. A further disadvantage of the inverter is that there is no way to provide independent control of the two output transistors.
A clock edge will appear to be less accurate if the clock edge is slow. This is because noise is always superimposed on the clock. The noise artificially moves the edge position forward or backward in time by temporarily shifting the voltage on the clock distribution. If the edge transition can be made faster, then the apparent accuracy of the clock improves, because faster clock transitions decrease noise-generated clock skew. With less skew, more machine cycle time is available to perform useful logic at a given frequency.
Accordingly, what is needed is a system and method for generating a faster clock edge. The present invention addresses such a need.